1. Field of the Invention
The present invention relates to a color processing method and a color processing apparatus, and more particularly to a process of mapping input colors into an output color gamut.
2. Description of the Related Art
With the popularization of personal computers and workstations in recent years, desktop publishing (DTP) and computer-aided design (CAD) are in general and widespread use. Under the circumstances, color reproduction technology becomes significant to perform matching colors expressed on a monitor by a computer with colors reproduced on an output medium. In DTP, for example, a color image process advances in such a work flow as creating, editing, and processing a color image on a monitor, and finally outputting the image using a color printer. Users strongly desire that color images on the monitor are perceptually coincident with the images output from the printer in this work flow.
However, in color reproduction technology, there has been difficulty in having a color image on the monitor perceptually agree with an output image from the printer. This difficulty arises for the following reasons.
On the color monitor, a color image is formed by using phosphors to emit light of specific wavelengths. On the other hand, in the color printer, a color image is formed by using ink or the like to absorb light of specific wavelengths and reflecting remaining light. Because of the difference in producing an image as described, the color reproduction range differs greatly between the color monitor and the color printer. Moreover, even among the color monitors, the color reproduction range differs between a liquid crystal monitor and an electron-gun cathode-ray tube monitor. Again, even in the color printers, the color reproduction range differs due to a difference in paper quality or a difference in ink composition, for example. As has been described, since there is such diversity in the color reproduction capacity, it is impossible to achieve complete colorimetric matching in color between images on the monitor and images from the color printer or between color output images from different printer models or among different-quality paper materials.
Technology for absorbing the difference in color reproduction range and for perceptual agreement in display images among display media of different color reproduction ranges includes gamut mapping technology. This gamut mapping technology is a technology of mapping a given color reproduction range to another color reproduction range in L*a*b* color space or JCH color space, for example. In gamut mapping technology, there are different methods as follows.
(C1) A method of mapping a whole color reproduction range in a linear fashion.
(C2) A method of mapping only colors outside a color reproduction range to the surface of the color reproduction range.
(C3) A method of, while compressing original colors as little as possible in a color reproduction range, compressing colors outside the color reproduction range to a high saturation portion of the color reproduction range with one point on the L axis used as the target point in order to keep gradation (U.S. Pat. No. 6,041,136).
With the method (C1) mentioned above, because the whole input color gamut is mapped, gradation is prevented from being impaired. However, all of the colors, including representative colors, will change.
With the method (C2) mentioned above, it sometimes occurs that two points outside the color reproduction range are mapped at the same position on the outer periphery of the color reproduction range. This causes gradation to be lost and results in a loss of information that an image possesses.
In the method (C3) mentioned above, it sometimes occurs that the representative colors, calculated based on a perceptual correspondence relation between the monitor and the printer, are also mapped. As result, a considerable disagreement arises in the human perception of the representative colors. This problem also exists in the method (C1) mentioned above.